The present invention relates generally to computer system backplanes and, more particularly, to powerplanes for distributing power in backplanes.
A computer system backplane typically is a multi-layer substrate comprising a plurality of conductive layers interweaved with a plurality of dielectric layers. The backplane carries a plurality of parallel multiterminal sockets that receive in an edgewise manner circuit boards on which computer system components are constructed. Some of the backplane conductive layers are used for signal propagation. Other conductive layers are used to distribute the power necessary for system operation. These conductive layers are known in the art as powerplanes and are generally in the form of solid sheets of conductive material such as copper.
Each multiterminal socket typically includes a plurality of pins which pass through small, plated vias bored through the layers of the backplane. Each pin makes contact with a desired one of the backplane conductive layers. Where no connection to a particular conductive layer is desired, a region surrounding the via through that conductive layer is insulated to prevent the pin from making contact. The plated vias are sized relative to the connector pins for a press fit. Power supply connections are made in a generally similar manner.
In each powerplane, some of the plated vias make contact with load pins, i.e., pins coupled to the circuit boards received by the sockets. Other vias are connected to source pins coupled to a power supply.
It will be appreciated that due to design constraints the source pins are not always centered between the load pins, leading to unequal distribution of current over the powerplanes and unequal current sharing among the load pins. For example, load pins having a shorter linear distance to the source pins will have a lower resistance with respect to the source pins and thus will source more current than load pins further from the source pins. To avoid exceeding the current rating of the load pins closest to the power source, smaller power levels are required. This results in inefficient use of the current sourcing capacity of the for distant load pins.
An attempt to equally distribute current has been made using a stepped backplane configuration. See U.S. Pat. No. 4,450,029 to Holbert et al. In a stepped backplane, the conductive and dielectric layers are laminated while having the same transverse extent. An edge of the backplane is then milled to expose the conductive layers in a stepped fashion. Rectangular bus bars are then mounted to the exposed conductive layers to provide a parallel power distribution. The step backplane however fails to provide equal current over the length of the powerplanes and thus fails to provide equal current to each load pin. Moreover, stepped backplanes are costly as they require post lamination milling.
Consequently, there exists in the industry a need to provide a cost effective method for evenly distributing current to the load pins of a powerplane. The present invention addresses this need as well as other needs.
The present invention is a powerplane for use in a backplane power distribution system. The backplane includes a conductive sheet for distributing power from a power source to a load. The powerplane further includes source locations and load locations for coupling the conductive sheet to a power source and a load. The conductive sheet is provided with impedance variations for balancing the resistance of the conductive sheet between the source locations and load locations, thereby promoting even distribution of current to the load locations.